Asymmetrical current steering output driver with compact dimensions

ABSTRACT

A current output circuit with two output nodes connectable to a load and providing a plurality of discretely selectable current output magnitudes is provided. The circuit consists of a current driver attached across the output nodes supplying a particular current and one or more bypass resistors connected in parallel with the output nodes that can be switched between a non-conducting state and a resistive conducting state. When a load is connected across said output nodes, the particular magnitude of current sourced through the load can be selected by switching the state of the bypass resistors. The magnitude of the bypass resistors is preselected to provide several discretely selectable states of output current of substantial equal steps. The current driver can be combined with a current switch to increase the number of output states. Several current drivers with current switches may be provided and the bypass resistors integrated into the current switch itself.

TECHNICAL FIELD

This invention relates to a current mode output circuit. More particularly, this invention relates to an output circuit with a discretely variable output current.

BACKGROUND OF THE INVENTION

One type of data interface uses changes in the magnitude of a current to transmit data signals. In the simplest case, one state is represented by the presence of a current flow and another by the absence of current. An output circuit of this type using a switch S to connect a current driver through the output nodes to a load R_(L) is shown in FIG. 1a.

Other conventional data interfaces, such as those used in SCSI architecture, require a current to be always present on an active interface. The direction of current flow indicates whether the transmitted signal is a logic 0 or a logic 1. A conventional current steering output buffer of this type is illustrated in FIG. 1b. The circuit contains a current driver which is connected to the output nodes by a current switch. In a first state, switch pair S_(A) is closed, switch pair S_(B) is open, and the current driver contributes a current I to the total load current. In a second state, switch pair S_(B) is closed, switch pair S_(A) is open, and the current driver contributes a current of −I to the total load current. In conventional systems, current may be turned off by opening both switch pairs S_(A) and S_(B).

As performance of integrated circuits continues to increase, the limited number of I/O (input/output) pins demands better utilization. Incorporating an output signal with more than the two conventional digital output states over a wire allows the interconnect of the I/O to carry a larger data bandwidth. Previous attempts to do this have used signalling via multiple voltage levels. However, in many circumstances, it is preferable to use current signaling instead. Further, conventional circuits generate switching noise at either of the power supply leads as the amount of power flowing into the output circuit is continuously changed to thereby vary the output signal level.

SUMMARY OF THE INVENTION

According to the present invention, an output buffer is provided that utilizes current signaling to provide a multi-level data output and draws a constant amount of current from the power supply regardless of the output level, thus reducing switching noise and startup delays. An output buffer circuit according to a first embodiment of the present invention has a current driver supplying a current I which is connectable to a load resistance. The connection may be through a current switch. The circuit also has one or more bypass resistors which can be switchable connected in parallel with the load resistance or disconnected from the circuit. When a bypass resistor is switched in parallel with the load resistor, some of the output current is diverted through the bypass resistor, thus reducing the current supplied to the load. For N switched bypass resistors, there are 2^(N) possible resistor state combinations. By choosing appropriate resistance values for the bypass resistors, the current through the load resistor may be varied in 2^(N) discrete. The number of states may be increased to 2^(N+1)+1 if a current switch which allows the direction of the output current to be reversed or stopped is included in the output circuit. According to the invention, a zero-current output state is achieved by closing all switches in the current switch, instead of opening them. In this state, the introduced current flows equally in both directions through the load and therefore does not contribute a net current even though the given current driver is still sourcing current. Because the effect of a current driver on the load current may be eliminated without shutting off the current, it is possible to avoid startup delay and noise which would be introduced if the current switch were disconnected from the circuit by opening all the internal switches. Preferably, N bypass resistors are provided having appropriate resistances to provide at least N equal current steps for each current direction.

In another embodiment, a plurality of current drivers is provided and connected to the load resistance in parallel. It can be appreciated that when both of the current paths in a given current switch are active (e.g., all internal switches in a particular current switch are closed), no net current flow will be introduced into the load. However, the internal switches will also introduce a resistance in parallel with the load resistor. According to this aspect of the invention, the resistance of each internal switch is chosen so that when all internal switches are closed, the current switch provides a preselected effective resistance in parallel with the load which diverts current flow from other (active) current drivers and thereby reduces the current in the load by a predefined discrete amount. The magnitude of each current source and the effective resistance of the switches in each current switch can be chosen to minimize the number of redundant output states, and therefore maximize the number of possible output current steps.

A multiple-state current output circuit according to the present invention can be fabricated as an integrated circuit using MOS transistors and located on the same chip as its driving circuitry. The present design allows for compact circuit dimensions when compared with conventional circuits of a similar type.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other features of the present invention will be more readily apparent from the following detailed description and drawings of illustrative embodiments of the invention in which:

FIGS. 1a and 1 b are ideal representations of conventional current output circuits;

FIG. 2 is an ideal representation of an output circuit according to a first embodiment of the invention;

FIGS. 3a and 3 b are graphs of load current for various switch states and resistance values in the circuit of FIG. 2;

FIG. 4 is an ideal representation of an output circuit according to a second embodiment of the invention;

FIGS. 5a and 5 b are circuit diagrams of switchable bypass resistors according to the invention;

FIGS. 6a-6 c are ideal representations of an output circuit according to a third embodiment of the invention;

FIGS. 7a-7 c show circuit diagrams of the preferred implementation of the circuits of FIGS. 6a according to one specific implementation of this particular embodiment; and

FIG. 8 is a graph of the output currents for the circuit of FIGS. 7a-7 c with a varying transistor bias voltage.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 is an ideal representation of an output circuit according to a first embodiment of the invention having two output nodes 20 and 22 and a current driver comprised of current source 24 which supplies a current I. The output driver is connected to a receiving unit, indicated as load resistance R_(L) 26, through the output nodes 20, 22. According to the invention, one or more bypass resistors R₁ through R_(N) are switchable connected in parallel with the load resistance R_(L), e.g., by means of switches S₁ through S_(N), to form switchable bypass resistor assemblies. By closing one or more of the switches, the current I_(L) driven through the load 26 can be varied in discrete steps between a maximum current of I and a minimum current of I_(L)=I*R_(E)/(R_(L)+R_(E)), where R_(E) is the equivalent parallel resistance introduced when all N bypass resistors are conducting. Advantageously, the net current flow from the current driver 24 remains constant at I, even as the load current is changed. This reduces transient switching effects on the current driver and allows for high switching speeds and low output noise.

The value of bypass resistors R₁ through R_(N) can be defined in terms of the expected load resistance R_(L) and preferably are chosen so that the output buffer can source current in at least N+1 substantially equal output current steps. For example, if N=3, the value of the bypass resistors can be selected to provide, for example, four equal output current steps of magnitude I, 3/4I, 1/2I, and 1/4I. Depending on various design considerations, such as switching speed, device area, etc., several switching configurations may be used. In a “single-switch” configuration, four equal steps are achieved by switching only one bypass resistor at a time to a conducting state. In tabular form, this particular “single-switch” arrangement is summarized as:

S₁ S₂ S₃ I_(L) 0 0 0   I 1 0 0 ¾I 0 1 0 ½I 0 0 1 ¼I

where a “1” indicates that a switch is closed, i.e., the bypass resistor is conducting, and a “0” indicates that the switch is open, i.e., the bypass resistor is non-conducting. To achieve this result, the values of the bypass resistors are:

R₁=3R_(L) R₂=R_(L) R₃=1/3 R_(L)

In the general case of M equal output current steps separated by I/M in a “single-switch” configuration, N=M−1 switched bypass resistors are provided. The values of each switched bypass resistor R_(n) and the resulting load current I_(L) when only that particular resistor is conducting (only switch S_(n) is closed) is given by equation 1: $\begin{matrix} {R_{n},{{I_{L{(n)}}\overset{N}{\underset{n = 1}{|}}} = {\frac{N - n + 1}{n}R_{L}}},{\frac{N - n + 1}{N + 1}I}} & \left( {{Equ}.\quad 1} \right) \end{matrix}$

Thus, for 8 equal states, N=7 and the values of bypass resistors R₁ to R₇ is:

7/1R_(L) 6/2R_(L) 5/3R_(L) 4/4R_(L) 3/5R_(L) 2/6R_(L) 1/7R_(L)

The output current when only bypass resistor R_(n) is conducting for n=1-7 is:

7/8I 6/8I 5/8I 4/8I 3/8I 2/8I 1/8I

(The final state, output of I, is achieved when all resistors are non-conducting, i.e., no switches are closed).

Other configurations may also be used to achieve equal output current steps. For example, in a “cumulative parallel” configuration, the switches are closed successively, adding more than one bypass resistor in parallel with the load resistor at a time. The four states achieved in this configuration where N=3 may be represented as:

S₁ S₂ S₃ I_(L) 0 0 0   I 1 0 0 ¾I 1 1 0 ½I 1 1 1 ¼I

To achieve this result, the values of the bypass resistors are configured as:

R₁=3R_(L) R₂=3/2 R_(L) R₃=1/2 R_(L)

It can be appreciated that even though the resistor values may be optimized to provide equal current steps for certain switch configurations, such as the single-switch or cumulative-parallel configurations discussed above, all of the 2^(N) switch combinations can be utilized if desired. FIG. 3a is a graph of the output current I_(L) for each of the eight possible output states using the values of the single-switch configuration example. FIG. 3b is a similar graph for the resistor values of the cumulative-parallel configuration example. In each graph, the points at equal steps of 1/4I are indicated by a box. In both graphs, it can be seen that when the bypass resistor values are configured to achieve four equal steps as discussed above, the minimum separation between current points diminishes as more resistors are switched into place. Accordingly, the number of additional detectable states may be limited by noise concerns. Therefore, it would be advantageous to introduce additional states without reducing the current point separation.

FIG. 4 shows the output driver 10 according to a second embodiment of the invention. In this invention, a current switch 15 is integrated with the driver of FIG. 2. Current switch 15 connects the current driver, here formed of matched current source 24 and current sink 25, to the output nodes 20, 22 via internal switch pairs S_(A) and S_(B). Although all four internal switches in a given current switch can be independently controlled, for purposes of this discussion, switches which are normally closed together are given the same designation and are discussed in pairs. When switch pair S_(A) is open and switch pair S_(B) is closed, current flows out from node 22 through the load 26 and back into node 20. When switch pair S_(A) is closed and switch pair S_(B) is open, current flows through the load in the opposite direction.

By including a current switch in the output circuit, the number of available current output states is doubled. In addition, a zero-current state can be achieved by opening or closing the internal switch pairs S_(A) and S_(B) in unison. Thus, for example, by using three switched bypass resistors configured as discussed above combined with a current switch as shown, eight primary output current states at steps of 1/4I can be achieved. An additional, ninth zero-current state can also be achieved by opening or closing both switch pairs S_(A) and S_(B) together.

Accordingly, in this example, a three-bit digital output may be encoded in a single current signal. Advantageously, the circuit according to the present invention provides a wide and uniform separation between each of the current levels. This results in a robust output signal which can absorb a significant degree of noise, up to +/−1/8I, and still provide extractable data.

In the preferred embodiment, the conducting or non-conducting state of the bypass resistors (here shown as the state of resistor switches S₁ to S₃) and the state of current switch pairs S_(A) and S_(B) is controlled by conventional logic circuitry which converts a three-bit output value into appropriate control signal combinations. If the bypass resistors R₁ to R₃ are configured to be used in a single-switch arrangement, as discussed above for example, the nine output logic states may be encoded as follows:

Output S₁ S₂ S₃ SA SB I_(L) 111 0 0 0 1 0   +I 110 1 0 0 1 0 +¾I 101 0 1 0 1 0 +½I 100 0 0 1 1 0 +¼I — — — — both 1/0 0 011 0 0 1 0 1 −¼I 010 0 1 0 0 1 −½I 001 1 0 0 0 1 −¾I 000 0 0 0 0 1   −I

A similar state table can easily be configured for the parallel-cumulative example discussed above. Appropriate logic circuitry can be designed directly from state tables such as this as is well known in the art.

FIGS. 5a and 5 b show preferred structures for implementing the switchable bypass resistors. In FIG. 5a, a switchable bypass resistor is implemented as two series-connected MOS transistors 100, 102 driven by a reference voltage Vref 103 which is adjusted so that the transistors 100, 102 are either non-conducting or resistively conducting. Preferably, a capacitor 104 is connected from node 105 (the junction of transistors 100, 102) to ground. Capacitor 104 serves to provide an AC ground for common mode signals which may be induced in the circuit. Although only one MOS transistor is necessary, preferably two are used in combination with a capacitor because it is beneficial to have an AC ground at the center point.

In this embodiment, when the driving voltage Vref 103 is below the turn-on voltage for transistors 100, 102, the transistors are non-conducting. When a voltage greater the turn-on voltage (for NMOS transistors) is applied to the gates of transistors 100, 102, they conduct and have a specific resistance dependant on the geometry of each transistor and the applied voltage gate Vref 103. The proper biasing voltage to apply to a given MOS transistor so as to set it to a particular predefined resistance can be generated using feedback circuits. One such feedback circuit is described below.

According to another aspect of the invention, the output circuit is fabricated as an integrated circuit on a single chip so that all transistors in the circuit are exposed to the same fabrication conditions. A single bias voltage Vref may then be used for all of the switched bypass resistors implemented as MOS transistors by scaling various transistors relative to a “master” transistor that is biased to a known resistance. For example, if transistor 100 biased with a voltage V has a resistance R, a resistance of 2R may be achieved by applying the same biasing voltage V to a transistor with a channel width half that of transistor 100 and having the same length. Similarly, a resistance of 0.5R may be achieved by applying voltage V to a transistor having a channel twice as wide but the same length as transistor 100. Because all the transistors are fabricated with identical process conditions, errors introduced during fabrication cancel out.

FIG. 5b shows an alternative embodiment for implementing a switched bypass resistor. In this embodiment, two resistors 116, 118 are connected in series with two MOS transistors 110, 112. The transistors are driven by a biasing voltage Vbias 114. Preferably, a capacitor 120 is connected between the junction of the transistors 110, 112 (node 119) and ground. As above, in the “off” condition, the transistors 110, 112 are non-conducting. However, in the “on” state, the transistors are driven with a biasing voltage sufficient to place them in a saturated state so that their internal resistance is small when compared with resistors 116, 118. In this embodiment, the transistors are preferably driven using digital signals of appropriate magnitudes. The resistors can be fabricated using any appropriate technique and can be on- or off-chip passive devices, or even appropriately biased transistors.

FIG. 6a is an ideal representation of a third embodiment of the invention. As shown, current output circuit 50 consists of a plurality of current drivers, in this example drivers 56, 57, 58, which are connected in parallel to output nodes 52, 54, and thereby to a load resistor R_(L) 26, representing an attached receiving device. Each of the current drivers preferably includes a matched current source and sink connected to the output nodes by a current switch. Driver 56 includes current source and sink 60, 62 which provide a current I₁ through current switch 64 having switch pairs S₁ and S₂. Driver 57 includes current source and sink 66, 68 which provide current I₂ through current switch 70 having switch pairs S₃ and S₄. Driver 58 includes current source and sink 72, 74 which provide current I₃ through current switch 76 having switch pairs S₅ and S₆.

According to this aspect of the invention, the output current I_(L) through load 26 may be varied by selectively providing various combinations of currents _(I1) to _(I3), in either direction, via the current drivers 56, 57, 58. Further according to the invention, the resistance of switch pairs S₁-S₆, when conducting, is set to various predefined values. Each switch thus functions as a switchable bypass resistor as discussed above. In addition, all of the internal switches in a given current switch are configured so that the net resistance for each path between the given current source and current sink is equivalent. This ensures that when all internal switches in a given current switch are closed, half the switched current flows in one direction through the load and half flows in the other direction.

In this state, no net current is introduced into the load from the particular current driver and therefore, the particular switched current can be ignored. According to the invention, however, because the closed internal switches have a predefined resistance, closing both switch pairs also introduces a predetermined effective resistance in parallel with the load. This diverts current flow from other current drivers and thereby reduces the current in the load by a particular amount. The resistances of the internal switches and thereby the effective resistance associated with each current driver is chosen to provide for a plurality of discretely selectable current levels through the load in a manner similar to that discussed above with respect to FIGS. 2-4.

Each internal switch in a switch pair S can be considered an ideal switch in series with a resistor R or a voltage-controlled resistor. For purposes of clarity, when a given switch is open, i.e., when the resistor is not conducting, only the open-switch portion is shown in the figures. When an internal switch is closed, i.e., the resistor is conducting, it is represented as the equivalent resistance. It can be appreciated that each internal switch in each of the current switches 64, 70, 76 can have a different internal resistance subject to the constraint, in this embodiment, that the two current paths in a given current switch, i.e., the path through each pair of switches, have equivalent resistances to allow for a zero net current flow state as previously discussed. For purposes of simplicity, it is assumed that each of the four internal switches comprising a given current switch 64, 70, 76 have the same internal resistance, defined as R₆₄, R₇₀, R₇₆, respectively.

FIG. 6b is an illustration showing the circuit 50 with switch pairs S₁, S₃, and S₅ closed and replaced with their equivalent switch resistances, R₆₄, R₇₀, R₇₆. Because each resistance is introduced in series between a current source/sink and the load, the internal resistances do not affect the net current flow, only the voltage drop across the load. Thus, the net current flow I_(L) through load resistor R_(L) is I₁+I₂+I₃. This sum is defined as the maximum output current I. (Analogously, if switch pairs S₂, S₄, and S₆ were closed, the total current through the load would be −I).

FIG. 6c is an illustration showing the circuit 50 with switch pairs S₃, and S₅ closed and replaced with their equivalent switch resistances, R₇₀, R₇₆. In addition, current switch 64 has both switch pairs S₁ and S₂ closed and is therefore in a zero net current state. Therefore, current I₁ is not contributing current to the load because the effect of switched current I₁ is equal in both directions. Note, however, that the I₁ current is still flowing, and therefore, switching noise and start-up delays associated with stopping current flow are not introduced. The total contributing current flow is I₂+I₃. However, as shown in the figure, the internal switch resistances from current switch 64 are now in parallel with the load resistor R_(L). This introduces a parallel aggregate resistance of (R₆₄+R₆₄)¦¦(R₆₄+R₆₄)=R₆₄, where “¦¦” signifies a parallel combination. This bypass resistance diverts a portion of the I₂+I₃ current, thereby reducing the output current I_(L). According to the invention, the parallel equivalent resistance (and thus each internal switch resistance) R₆₄ is chosen to provide a particular output current in a manner similar to that discussed above for FIGS. 2-5. Preferably, the switchable resistors used in switch pairs S₁-S₆ are implemented as MOS transistors as shown in FIGS. 5a and 5 b, discussed above, as are the current drivers 60, 66, and 72, and the reference voltage circuit which applies the appropriate gate voltage(s).

In one configuration, for example, I₁=0.1I, I₂=0.3I, and I₃=0.6I and the resistances R₆₄, R₇₀, and R₇₆ are set to 2R_(L), 2/3 R_(L), and 1/3 R_(L), respectively. When all three current switches 64, 70, and 76 are contributing current in the same direction (FIG. 6b), the net current flow is I. When current switch 64 is set to a non-contributing state as shown in FIG. 6c, current I₁ does not contribute to the load and a resistance of R₆₄=2R_(L) is placed in parallel with the load resistance R_(L). Because currents I₂ and I₃ remain contributing, the total sourced current is 0.3I+0.6I=0.9I. R_(L) is in parallel with a bypass resistance equal to 2R_(L) and so the current sourced to the load is 0.9I*2R_(L)/(R_(L)+2R_(L))=0.9I*2/(1+2)=0.6I. Similarly, in the case where switch pairs S₁, S₂, S₃, S₄, and S₅ are closed, only the current driver 58 is contributing I₃=0.6I and the resulting load current is 0.2I. Similar calculations can be performed for other current switch states as well. One group of switch states for this embodiment and the resulting load current is summarized in the following table, where a 1 indicates that a switch pair is closed and a 0 indicates that a switch pair is open.

S₁ S₂ S₃ S₄ S₅ S₆ I_(L) 1 0 1 0 1 0 I 1 1 1 0 1 0 0.6I 1 1 1 1 1 0 0.2I 1 1 1 1 1 1 0 1 1 1 1 0 1 −0.2I 1 1 0 1 0 1 −0.4I 0 1 0 1 0 1 −I

FIGS. 7a-7 c show circuit diagrams of a preferred implementation of this particular embodiment, using the selected values from the previous example. All components except for a single reference resistor are implemented as MOS transistors. All transistors preferably have the same gate length, most preferably the minimum value supported in a given fabrication technology, but may have varying gate widths. According to one aspect of the invention, the entire output circuit is fabricated as part of a single integrated circuit where the current output circuits are simply scaled versions of each other. In a simple scaling scheme, the lengths of the transistor gates are kept constant, while the widths of the gates are adjusted to provide the desired relative resistance and/or current.

Turning now to FIG. 7a, representative current driver 56 consists of a current source formed by p-channel MOS transistor 150 biased to a gate voltage Pch, and a current sink formed by N-channel MOS transistor 152 biased to a gate voltage Nch, which together produce current I₁. Transistors 150 and 152 have gate widths of 70 and 18 microns, respectively, and are connected to output nodes 154, 156 through a current switch 64 formed by N-channel transistors 160, 162, 164, and 166, having gate widths of 95, 50, 95, and 50 microns respectively. The gates of transistors 160 and 162 are connected to input voltage IN₁ and together correspond to switch pair S₁ shown in FIG. 6a. The gates of transistors 164 and 166 are connected to input voltage INN₁ and together correspond to switch pair S₂ shown in FIG. 6a. The input gate voltages IN₁ and INN₁ are set to either turn off the given transistors (i.e., zero volts), or bias them to a common voltage, Vcom, which places them in a resistive conducting state having the desired resistance.

FIG. 7b is one embodiment of a voltage reference circuit 168 for generating the bias voltages Pch and Nch for transistors 150 and 152. The circuit consists of a current source formed by P-channel transistor 170 and a current sink formed by N-channel transistor 172. The current driver formed by transistors 170 and 172 is connected to output nodes 174, 176 through a current switch formed by N-channel transistors 180, 182, 184, and 186, which are connected as shown. The gates of transistors 180 and 182 are driven by a bias voltage Vcom which places them in a resistive conducting state. The particular resistance of these switches is set by adjusting the value of Vcom as desired and will be discussed below. The gates of transistors 184 and 186 are grounded and therefore, these transistors are non-conducting.

A reference resistance R_(REF) 188 is connected across nodes 174 and 176. Resistor 188 is chosen to have a resistance equal to the expected load resistance R_(L) 26 which will be connected to the output buffer. The geometric dimensions of transistors 170, 172, 180, 182, 184, and 186 correspond symmetrically to transistors 150, 152, 160, 162, 164, and 166 respectively (FIG. 7a). Preferably, corresponding transistors have the same dimensions. Because the circuits of FIG. 7a and 7 b correspond geometrically, it can be appreciated that once the bias voltages for transistors 170 and 172 are set to provide a specific current through known resistance R_(REF) 188, i.e., a current equal to I₁, these bias voltages can then be applied to corresponding transistors in output buffer circuits 56 to produce currents with a predefined magnitude when driving a load having resistance equal to R_(REF).

The proper biasing voltages, Pch and Nch, are generated using feedback comparators 190 and 192. Comparator 190 is driven by input voltage V2 and the voltage at node 174 and its output is used to bias transistor 170. This circuit will adjust the biasing of transistor 170 until the sourced current places the voltage at node 174 equal to V2. Similarly, comparator 192 is driven by input voltage V1 and the voltage at node 176 and its output used to bias transistor 172. This circuit will adjust the bias of transistor 172 until the sunk current places the voltage at node 176 equal to V1. Since the reference resistance R_(REF) 188 is known, the voltage drop across the resistor 188 can be set to provide the desired current, i.e., I=(V2-V1)/R_(REF). For example, with a resistance of 100 Ohms, a set voltage drop between 2.4 and 2 volts would result in a current of 4 milliamps.

Voltage reference circuits of this type are described in more detail in commonly assigned U.S. patent application Ser. No. 08/882,827 entitled “Low Voltage Differential Swing Interconnect Buffer Circuit,” filed on Jun. 26, 1997, the entire contents of which are hereby incorporated by reference.

FIG. 7c shows a complete output circuit implementing the specific example discussed above and using the circuits of FIGS. 7a and 7 b. As shown, reference circuit 168 generates bias voltages Pch and Nch based on reference resistor 188 and input voltages V1, V2, and Vcom. In this example, voltages V1 and V2 have a difference of several hundred millivolts, i.e., V1=1.0 volts and V2=1.4 volts. A plurality of output drivers are connected in parallel between output nodes 52 and 54. The output circuit 56 shown in FIG. 7b corresponds to the “1x” output circuit. Circuits 57 and 58 are scaled versions of circuit 56, where the transistors have gate widths of 3x and 6x, respectively, of the corresponding transistors in circuit 56. Accordingly, if circuit 56 is defined to produce output current I₁=0.1I, circuit 57 will produce I₂=3*I₁=0.3I, and circuit 58 will produce output current I₃=6*I₁=0.6I. The switch states of each output driver are controlled by a digital control circuit 200, which implements, for example, the state table shown above by providing a zero or ground voltage to a given transistor gate when it is to be non-conducting and providing an output voltage equal to Vcom when a switch is to be in a conducting state. Control circuits of this type are well known in the art.

As mentioned above, varying the biasing voltage for conducting transistors (i.e., Vcom) results in different output currents because the resistance of the transistors is varied. FIG. 8 is a graph of simulated output currents for the example circuit discussed above with Vcom varied between 2.0 volts and 3.8 volts. As can be seen, results generally in line with the ideal situation are achieved with a driving voltage Vcom of approximately 2.2 volts. Various methods of producing the appropriate driving voltage are known to those skilled in the art and include, for example, resistive divider circuits, feedback circuits, etc.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A current output circuit comprising: first and second output nodes; a separate load resistance connectable to said first and second output nodes; a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load resistance when said load resistance is connected to said output nodes; wherein, when the load resistance is connected to said first and second output nodes, the total current flow from the first output node to the second output node through said load resistance and any conducting bypass resistors is substantially I for each of the discretely selectable output current states.
 2. The circuit of claim 1, wherein the value of each predetermined resistance is such as to provide a plurality of substantially equal magnitude output current steps between output current states through said load resistance.
 3. The circuit of claim 2, comprising N bypass resistors, the predetermined resistance of said bypass resistors relative to the expected load resistance being such as to provide N+1 selectable output current states with current magnitudes varying between approximately (1/N+1)*I and I in discrete steps of substantially (1/N+1)*I.
 4. The circuit of claim 1, further comprising a current switch connected between said current driver and said output nodes to control the direction of current flow through said load, when connected.
 5. The circuit of claim 1, wherein each said bypass resistor comprises an MOS transistor controlled by a voltage applied at the gate thereof, said MOS transistor being non-conducting when the magnitude of said gate voltage is a first voltage and said MOS transistor being conducting and having a predetermined resistance when the magnitude of said gate voltage is a second voltage.
 6. A current output circuit having first and second output nodes which are connectable to a separate load resistance, said circuit comprising: a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load when said load is connected to said output nodes; wherein, when the load resistance is connected to the circuit, the current flow through the load and any conducting bypass resistors from the first output node to the second output node is substantially I for each of the discretely selectable output current states; wherein each bypass resistor comprises first and second series connected MOS transistors, the gates of said transistors being connected to a common reference voltage, said MOS transistors being non-conducting when the magnitude of said reference voltage is at a first voltage, said MOS transistors being conducting and having predetermined resistance when the magnitude of said reference voltage is at a second voltage.
 7. The circuit of claim 6, further comprising a capacitor connected to the connection between said first and second transistors and to a common ground voltage.
 8. The circuit of claim 1, wherein each bypass resistor comprises a switch in series with a resistor.
 9. A current output circuit having first and second output nodes which are connectable to a separate load resistance, said circuit comprising: a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load when said load is connected to said output nodes; wherein, when the load resistance is connected to the circuit, the current flow through the load and any conducting bypass resistors from the first output node to the second output node is substantially I for each of the discretely selectable output current states wherein each said bypass resistor comprises: a first resistor; a first MOS transistor connected in series with the first resistor; a second MOS transistor connected in series with the first transistor; and a second resistor connected in series with the second transistor; the gates of the first and second transistors being connected to a common switching voltage, the transistors being non-conducting when said switching voltage is at a first voltage, and the transistors being conducting when said switching voltage is at a second voltage.
 10. An integrated circuit including: a current output circuit having first and second output nodes; a separate load resistance connectable between said first and second output nodes; a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load resistance when said load resistance is connected to said output nodes; wherein, when the load resistance is connected to the first and second nodes of the circuit, the total current flow from the first output node to the second output node through the load resistance and any conducting bypass resistor is substantially I for each of the discretely selectable output current states.
 11. The circuit of claim 10, wherein the value of each predetermined resistance is such as to provide a plurality of substantially equal magnitude output current steps between output current states through said load resistance.
 12. The circuit of claim 11, comprising N bypass resistors, the predetermined resistance of said bypass resistors relative to the expected load resistance being such as to provide N+1 selectable output current states with magnitudes varying between approximately (1/N+1)*I and I in discrete steps of substantially (1/N+1)*I.
 13. The circuit of claim 10, further comprising a current switch connected between said current driver and said output nodes so as to control the direction of current flow through said load.
 14. The circuit of claim 10, wherein each said bypass resistor comprises a transistor.
 15. The circuit of claim 10, wherein each bypass resistor comprises an MOS transistor controlled by a signal switchable between first and second voltage magnitudes, said MOS transistor being non-conducting when the magnitude of said signal is at a first voltage and said MOS transistor being conducting and having a predetermined resistance when the magnitude of said signal is at a second voltage.
 16. The circuit of claim 15, wherein a particular MOS transistor in one of said bypass resistors is a scaled version of a corresponding MOS transistor in a second one of said bypass resistors, the controlling signals for said scaled and corresponding MOS transistors being switchable between substantially the same first and second voltages.
 17. An integrated circuit including a current output circuit having first and second output nodes which are connectable to a separate load resistance, said current output circuit comprising: a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load when said load is connected to said output nodes; wherein, when the load resistance is connected to the circuit, the current flow through the load and any conducting bypass resistor from the first output node to the second output node is substantially I for each of the discretely selectable output current states; wherein each bypass resistor comprises first and second series connected MOS transistors, the gates of said transistors being connected to a common reference voltage, said MOS transistors being non-conducting when the magnitude of said reference voltage is at a first voltage, said MOS transistors being conducting and having a predetermined resistance when the magnitude of said reference voltage is at a second voltage.
 18. The circuit of claim 17, further comprising a capacitor connected to the connection between said first and second transistors and to a common grounding voltage.
 19. The circuit of claim 10, wherein each bypass resistor comprises a switch in series with a resistor.
 20. The circuit of claim 10, wherein each bypass resistor comprises: a first resistor; a first MOS transistor connected in series with the first resistor; a second MOS transistor connected in series with the first transistor; and a second resistor connected in series with the second transistor; the gates of the first and second transistors being connected to a common switching voltage, the transistors being non-conducting when said switching voltage is at a first voltage, and the transistors being conducting when said switching voltage is at a second voltage.
 21. A current output circuit having first and second output nodes connectable to a load resistance, said circuit comprising: a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load when said load is connected to said output nodes; each said bypass resistor comprising: a first resistor, a first MOS transistor connected in series with the first resistor, a second MOS transistor connected in series with the first transistor, and a second resistor connected in series with the second transistor, the gates of the first and second transistors being connected to a common switching voltage, the transistors being non-conducting when said switching voltage is at a first voltage, and the transistors being conducting when said switching voltage is at a second voltage; wherein, when the load resistance is connected to the circuit, the current flow through the load and any conducting bypass resistors from the first output node to the second output node is substantially I for each of the discretely selectable output current states.
 22. The circuit of claim 21, wherein the value of each predetermined resistance is such as to provide a plurality of substantially equal output current steps between output current states.
 23. The circuit of claim 22, comprising N bypass resistors, the predetermined resistance of said bypass resistors relative to the expected load resistance being such as to provide N+1 selectable output current states with current magnitudes varying between approximately (1/N+1)*I and I in discrete steps of substantially (1/N+1)*I.
 24. The circuit of claim 21, further comprising a current switch connected between said current driver and said output nodes so as to control the direction of current flow through said load, when connected.
 25. An integrated circuit including a current output circuit having first and second output nodes connectable to a load resistance, said current output circuit comprising: a current driver connected to at least one of said first and second output nodes and supplying a total and substantially constant current I; and at least one bypass resistor connected between said first and second output nodes, each bypass resistor being switchable between a non-conducting state and a conducting state having a predetermined resistance, the states of said at least one bypass resistor providing a plurality of discretely selectable output current states of particular current magnitudes through said load when said load is connected to said output nodes; each said bypass resistor comprising: a first resistor, a first MOS transistor connected in series with the first resistor, a second MOS transistor connected in series with the first transistor, and a second resistor connected in series with the second transistor, the gates of the first and second transistors being connected to a common switching voltage, the transistors being non-conducting when said switching voltage is at a first voltage, and the transistors being conducting when said switching voltage is at a second voltage; wherein, when the load resistance is connected to the circuit, the current flow through the load and any conducting bypass resistor from the first output node to the second output node is substantially I for each of the discretely selectable output current states.
 26. The circuit of claim 25, wherein the value of each predetermined resistance is such as to provide a plurality of substantially equal output current steps between output current states.
 27. The circuit of claim 26, comprising N bypass resistors, the predetermined resistance of said bypass resistors relative to the expected load resistance being such as to provide N+1 selectable output current states with magnitudes varying between approximately (1/N+1)*I and I in discrete steps of substantially (1/N+1)*I.
 28. The circuit of claim 25, further comprising a current switch connected between said current driver and said output nodes so as to control the direction of current flow through said load. 